The preferred embodiment relates to a field device, such as a positioner, an I/P-converter or the like for a processing plant, as well as to a method of energy supply for a field device.
Field devices used for process automation of a processing plant can be divided into two categories. These are on the one side field devices receiving and modulating a processing signal for providing a control signal, in particular a positioning signal. Such field devices are also referred to as output devices. Examples for field devices designed as output devices are the positioner or the current/pressure converter (I/P-converter). On the other hand, field devices can also be used to measure a physical quantity for a process automation system, such as temperature, pressure or flow rate, and are referred to as transducers. In the following, the preferred embodiment is described primarily with reference to the former, that is output devices such as positioners or I/P-converters. It is to be understood that the preferred embodiment can also relate to transducers.
For a simple representation and explanation in particular of an input side of a generic, known field device, see FIG. 1 showing a block diagram of the known field device.
Commonly, the field device a for process automation is constructed as follows. A known field device a can feature a pneumatic actuator b operating a control valve c by means of a drive rod, the valve being arranged within a duct that conveys a process fluid of the processing plant (not shown in the drawings). The field device a has an electronic unit d with an electrical signal output e connected to an I/P-converter f. The electronic unit d is connected with a position sensor g that transmits position signals of the drive rod to the electronic unit d via line h. The electronic unit d can feature a data storage unit and a micro-processor. The electronic unit d is preceded by an input circuit i that, together with a diode k, is used for limiting the input voltage of the electronic unit d. Furthermore, the input circuit i has a current measuring unit l as well as a measurement resistor RM at which the current of an input current signal i is measured. Subsequently, the current measuring unit l feeds the converted current signal to the electronic unit d.
The field device shown in FIG. 1 is a positioner system controlling a physical quantity, such as the flow rate of the processing plant. The field device a according to FIG. 1 can receive a process signal in the form of a current signal i from 4 to 20 mA. By means of a position signal of the position sensor g a control signal is calculated within the electronic unit d, which is fed to I/P-converter f. The I/P-converter f generates a pneumatic control signal that is fed to the pneumatic actuator b, which positions the control valve c accordingly.
The voltage present at the voltage limiting diode must be such that even in case of a zero point signal of 4 mA or slightly below (3.8 mA) sufficient energy is supplied to the electronic unit d. Allowing for the tolerances of the component parts, in particular the electronic unit d, a worst case situation must be considered in order to ensure that the provided voltage is always slightly higher than the voltage required by the electronic unit d. The unused energy is converted into heat in the voltage limiter of the diode k. Likewise for a higher current signal of for example 15 mA, all additional energy is dissipated into heat at the voltage limiting unit.
It shall be understood that the input circuit i can also be connected to a field bus. In this case, the field device is supplied with a constant current of for example 15 mA while the supply voltage may vary between 9 and 32V. Therein, a current controller may utilize a measurement resistor in order to measure the loop current, and drives a transistor such that the current is controlled to be constant. For a field bus connection, the current should be maintained constant because for the digital field buses in consideration, the current is modulated without a mean value in order to encode digital information.
Because of their wide ranging functionality and their high safety standards, positioners and also I/P-converters are often equipped with a number of electronic components or at least high quality electronic components. Positioners designed to fulfil multiple tasks comprise a micro-processor, possibly several data storage units, communication units such as wireless transmitters and receivers, and several sensing devices such as a position sensor.
In the area of process automation, there is a general interest to fit up field devices with further reaching additional functions such as functional diagnosis, self-surveillance, etc., whereby not only the capability of the field device is increased but also the number of components and the energy consumption of the field device.
However, for the known field devices, the available electrical energy is limited also because of ignition protection regulations.
There are efforts to increase the amount of electrical energy provided to the field device. For example, for the energy supply method known as Energy Harvesting energy already present, such as pneumatic drive energy or flow energy of the process medium of the processing plant is used. Therein, piezo-electric and thermo-electric energy exchange principals are applied. The Energy Harvesting method has the principal disadvantage of high technical expenditure, especially in view of the necessary ignition protection and a precise energy conversion technique.
For example from DE 10 2006 011 503 a process automation field device such as a positioner is known that is fitted with an energy converter converting flow energy of the process fluid into electrical energy. DE 10 2004 004 930 A1 also discloses an energy conversion unit for which a mechanical pendulum is exposed to a pneumatic power source, thereby stimulated to oscillate, and the oscillations are converted into electrical energy.
DE 10 2004 059 106 B4 suggests a further method for providing electrical energy to a field device by means of energy conversion. For supplying energy to a field device, a secondary winding is arranged around an electrical supply line of the processing plant. Owing to the induction fields around the electrical supply line, electrical energy can be provided to the field device.
A further method for providing energy to field devices is described in WO2006/127421 A2. There the field device is coupled to a HART field bus via a Zener diode. A power controller is provided acting as a negative resistance. For a constant supply current of for example 4 mA, the power controller causes an increase of the connected compliance voltage of for example 9V to 11V thereby providing an increased electrical power of 44 mW by the master controller. Such an additional energy supply has the disadvantage that the master controller of the processing plant, which for example is connected to the field device via a field bus, cannot supply sufficient electrical power in order to provide the increased compliance voltage. Furthermore, the control circuit for raising the supply voltage by means of the power controller has a complicated structure. Furthermore, a higher energy consumption must be assigned to the field device. Moreover, for the known additional energy supplies it is not guaranteed that the constant control current value is not negatively affected by the variation of the compliance voltage.